iOS Metro hint doesn't reach expo-dev-client apps (writes bare-RN RCT_jsLocation only)

[thymikee]

Summary

On iOS, agent-device's Metro/dev-server runtime hint (open --metro-host/--metro-port, metro reload, metro prepare) does not point an expo-dev-client app at the chosen Metro server. It writes bare-React-Native's RCT_jsLocation preference, which an expo-dev-client ignores — so the app keeps loading from its own dev-server URL (or embedded bundle) instead of the requested port.

This is the iOS analogue of the Android debug_http_host-wrong-file bug fixed in #1242: the intent is right, but the mechanism targets bare-RN while the app framework (expo-dev-client) reads a different source.

Root cause

applyIosSimulatorRuntimeHints in src/daemon/runtime-hints.ts sets RCT_jsLocation (+ RCT_packager_scheme) in the app's NSUserDefaults via simctl. Bare RN's RCTBundleURLProvider reads RCT_jsLocation, so this works for bare-RN debug apps. But an expo-dev-client resolves its dev-server from its own launcher state / the exp+<scheme>://expo-development-client/?url=<metro-url> deep-link, not RCT_jsLocation — so the hint is silently a no-op.

Evidence (verified live, iPhone 17 Pro simulator)

• After open com.callstack.agentdevicelab --metro-host 127.0.0.1 --metro-port 8085, defaults read com.callstack.agentdevicelab RCT_jsLocation returned 127.0.0.1:8085 — the hint was written correctly.
• The app nonetheless did not request a bundle from Metro on 8085 (Metro logged only "Waiting on http://localhost:8085"); it kept running its own bundle.
• The dev build does register the exp+agent-device-test-app URL scheme (the expo-dev-client launcher handler), confirming it is an expo-dev-client and that the exp+…://expo-development-client/?url= path is the intended mechanism.

Suggested fix direction

Detect an expo-dev-client target (it registers an exp+<scheme> CFBundleURLScheme in Info.plist) and, for those, point Metro via the expo mechanism instead of RCT_jsLocation:

• Launch/redirect via xcrun simctl openurl <udid> "exp+<scheme>://expo-development-client/?url=<url-encoded-metro-url>".
• agent-device already computes the correct expo bundle URL via metro prepare --kind expo (.expo/.virtual-metro-entry.bundle), so the pointing side is the gap.

Important caveat (separate limitation)

An embed-first dev build — one shipped with a baked main.jsbundle and configured to launch straight into the embedded bundle rather than the dev-launcher — runs the embedded bundle regardless of any metro hint or deep-link. That case genuinely requires a native rebuild (expo run:ios) and is not solvable by the fix above; only launcher-style expo-dev-clients benefit.

Context

Surfaced while capturing live over-filtering evidence for the replay-divergence chrome filter (PR #1233). Related: #1242 (the Android debug_http_host counterpart).